1. Field of the Invention
This invention relates to a microwave frequency synthesizer used as a local oscillator in a transmitter or receiver in satellite communication, for example.
2. Description of the Prior Art
In recent years, microwave frequency synthesizers have been prevalently used as local oscillators in transmitters or receivers in sattelite communication and as signal sources for evaluating microwave devices. These microwave frequency synthesizers are disclosed in the reference: "Frequency Synthesizers, Theory and Design" by Vadio Manasewitsch, Chapter 7, FIG. 7-30, for examples.
Now referring to FIG. 1, the principal theory of a previously known microwave frequency synthesizer will be explained below.
First, a phase comparator 1 compares a signal from a voltage control oscillator 6 divided by divider 5 with a signal from a reference oscillator 2 divided by divider 3 to take a phase lock therebetween. A voltage control oscillator 6 sends to a mixer 7 an oscillation signal corresponding to an output signal from loop filter 4. Mixer 7 adds the oscillation signal to the signal sent from reference oscillator 2 via a multiplier 8 and sends the result to a multiplier 9 where it is multiplied.
It is considered that in actual cases, an output of 6550 MHz to 7050 MHz is obtained at an terminal 10 by setting the frequency of reference oscillator 2 at 5 MHz, the multiplication factor of multiplier 8 at 81, and that of multiplier 9 at 20.
In the arrangement as mentioned above, however, the phase noise is degraded each multiplication, and for example, N time multiplication results in a phase noise degradation of 20 log N dB. Actually, with N=20, the phase noise is degraded by 26 dB so that the phase noise of -100 dB/Hz is degraded to that of -74 dB/Hz.
Further, in the arrangement with an increase in frequency division number for phase comparison, there has occured a problem that the noise produced from a frequency divider or phase-locked loop PLL.IC is enhanced in its floor and the comparison frequency in the loop leaks in the synthesizer output. If the comparison frequency in the loop cannot be set at a higher value, the low nosie cannot be realized by means of setting higher the natural frequency in the loop.